Donald Trump started whining about the presidential debates last weekend, perhaps setting the stage for skipping them. It should be a reminder that the tradition of televised presidential debates has always been something of a miracle -- one that didn’t have to become established, and one that could end fairly easily.

The first televised debates between U.S. presidential candidates took place in 1960, when Richard Nixon and John Kennedy faced each other four times. It wasn't until 1976 that the next general-election debates took place: President Gerald Ford debated Jimmy Carter three times. In 1980, President Carter debated Ronald Reagan only once. Since then, each election has had either two presidential debates or three.

The conventional wisdom is that debates are opportunities for the candidate who is losing, and risky for the one who is winning. Challengers are seen as benefiting from being on stage and from appearing on equal terms with an incumbent president.

In the case of Kennedy and Nixon, both were both running for president for the first time and were in a close election, so it made sense they would agree to debate. But in 1964, Lyndon Johnson, who had become president after Kennedy's assassination in November 1963, had no interest in facing his Republican challenger, Barry Goldwater.

What revived these encounters was a historical fluke: the back-to-back elections in 1976 and 1980 in which an incumbent president was in trouble. In 1976, Ford was in the even more flukish position of having been elected to nothing larger than a House of Representatives district before he became president after President Richard Nixon resigned.

The general-election debates were not an accepted custom until 1984, when President Reagan chose to face Walter Mondale twice. Perhaps Reagan and his campaign believed he had little to fear. It turned out he performed poorly in the first encounter, his only real scare in the campaign that year. But Reagan's willingness to take part in them in the first place established the debates as a regular part of the campaign. No subsequent candidate has been willing to risk the costs of breaking what is by now a long-observed tradition.

Until now possibly. True, Trump hasn’t threatened to back out so far. But speculation that he might doesn’t seem far-fetched. After all, while the reality-show host participated in several nomination debates, he dropped out of one before the Iowa caucuses, then refused to participate in any after March 10 (well before his last opponents ended their bids after the May 3 Indiana primary).

His specific complaint now -- that two scheduled debates are on nights with NFL games -- isn’t something to take seriously. As the debate commission has explained, the weeks before the election are full of NFL and college football games and the baseball postseason. With the commission committed to avoiding dates with Jewish holidays (which fall in October this year), conflicts with some major sports events are inevitable. The final 2012 debate between President Barack Obama and Mitt Romney went up against both Monday Night Football and the seventh game in the National League championship series.

But Trump can just bolt from the debates if he chooses to for any reason. And if he does, it’s going to make it that much easier for future presidential candidates to do the same thing. If so, we might have to wait a long time for another historical fluke to occur before they are re-established.

Debates are overrated when it comes to determining election outcomes. And the contention that elections should be settled through some one-on-one, high-pressure confrontation is not persuasive.

Nevertheless, debates have become an important part of representation. And violating norms is rarely a good idea since so much of U.S. democracy (and all democracies) depends on respecting customs, not on rules. If Trump bails, I hope the TV networks offer to give the time to Clinton anyway. And I hope they do it now, to create a strong incentive for Trump to participate.

Ford first replaced Vice President Spiro Agnew, who resigned in 1973, and became president a year later, when Nixon resigned.

This column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.